The recovery of data from a transmitted signal containing digital data, such as video and related information, at a digital receiver typically requires the implementation of three functions: timing recovery for symbol synchronization, carrier recovery (frequency demodulation) and equalization. Timing recovery is the process by which the receiver clock (timebase) is synchronized to the transmitter clock. This permits the received signal to be sampled at the optimum point in time to reduce the chance of a slicing error associated with decision-directed processing of received symbol values. In some receivers, the received signal is sampled at a multiple of the transmitter symbol rate. For example, some receivers sample the received signal at twice or four times the transmitter symbol rate. In any event, the sampling clock of the receiver must be synchronized to the symbol clock of the transmitter.
Carrier recovery is the process by which a received RF signal, after being frequency shifted to a lower intermediate frequency passband, is frequency shifted to baseband to permit recovery of the modulating baseband information. Equalization is a process which compensates for the effects of transmission channel disturbances upon the received signal. More specifically, equalization removes intersymbol interference (ISI) caused by transmission channel disturbances. ISI causes the value of a given symbol to be distorted by the values of preceding and following symbols. These and related functions are described in greater detail by Lee and Messerschmitt in Digital Communication (Kluwer Academic Press, Boston, Mass. USA).
Prior receivers required a relatively stable source of a sampling clock signal, yet controllable so that it could be locked to the transmitter symbol clock. Voltage controlled crystal oscillators (VCXOs) were used for this function. The clock signal produced by a VCXO is stable, but controllable over a relatively narrow range, so that it can be locked to the transmitter symbol clock. However, a VCXO is an analog component, so it is relatively expensive, and prone to drift over its lifetime. In addition, if it is necessary to receive signals from different transmitters having different symbol clock frequencies (such as in European satellite systems), it is necessary to have a separate VCXO for each such transmitter. further increasing the cost of the receiver.
An alternative timing recovery system disclosed in U.S. patent application Ser. No. 08/721,780, entitled "Timing Recovery System for a Digital Signal Processor" and filed Sep. 25, 19996 by Knutson et al. operates by sampling a received signal at a fixed frequency slightly higher than twice the highest transmitter symbol rate. These samples are then processed by an interpolator to generate a sequence of time interpolated samples synchronized to the transmitter symbol rate. These interpolated synchronized samples are supplied to a digital phase error detector. The output of the digital phase error detector is supplied to a second order loop filter. A predetermined value, representing a nominal sampling time delay, is added to the output signal of the loop filter. The combination of the predetermined nominal delay and the output signal from the loop filter controls a numerically controlled delay which provides integer and fractional clock delay component signals. The integer clock delay component signal is used to control production of a sampling clock enable signal synchronized to the transmitter symbol rate. This sampling clock enable signal may be further divided in frequency to provide a receiver symbol clock enable signal. The fractional clock delay component signal is applied to a control input of the interpolator so that the sampled signal produced by the interpolator represents the value of the received signal at the desired sampling time.
Such a timing recovery system can be used in systems carrying transmitted symbols QPSK modulated on a carrier. However, such a timing recovery system when arranged for QAM modulation is relatively complex and expensive. The required tolerances between the quadrature signals in a QAM signal with a dense constellation are difficult to meet. Errors introduced into the quadrature signals result in crosstalk between them which cannot be removed or reduced by an equalizer. A timing recovery system which can operate on QAM signals having differing symbol rates without undue complexity and expense is desired.